1. Field of the Invention
The present invention relates to equipment for the semi-continuous direct chill (DC) casting of sheet ingot or slabs of aluminium of different dimensions, in particular ingot or slabs for rolling thin sheet material, including a mold frame with a pair of facing side walls and a pair of facing end walls, the walls defining a mold with an upwardly open inlet for the supply of metal and a downwardly facing outlet provided with a starter block on a movable support which prior to each casting closes the downwardly facing opening and where at least one side wall and/or one end wall can be displaced to enable casting of ingots with different dimensions. The equipment further including means for cooling the metal and optionally means for flexing the mold to compensate for shrinkage.
2. Description of the Related Art
When casting large rectangular-section ingots to be used in the production of rolled products, it is customary to impart a small amount of convex curvature to the long side walls of the mold to counteract the greater metal shrinkage (pull-in) which takes place near the middle of the wide side faces of the ingot during solidification as compared with locations near the narrow end faces of the ingot. The shrinkage (pull-in) of the metal is proportional to the extension of the non-frozen metal in the ingot after casting conditions are stabilized. During the casting of large ingots the extension of molten metal in the lengthways direction of the ingot (the sump depth) may be up to 0.8 meter or more depending on the size of the ingot.
It is primarily the casting speed that influences the extension of the mushy zone, because it is the thermal conductivity of the material that limits the cooling speed in the middle of the ingot. The amount of water that is jetted onto the ingot surface on the underside of the mold represents a cooling capacity that goes beyond the amount of heat that is transferred to the surface by heat conduction.
With regard to both metallurgy and productivity it is desirable to apply the highest casting speed possible. The casting speed is normally limited by the tendency of hot crack formation in the ingot being cast when the speed is too high.
In the initial stage of a casting operation the cooling will be slow and there will be a contraction in the ingot being cast caused by the difference in specific density between the melted and the frozen metal, together with the thermal coefficient of expansion. The metal that initially has frozen will be of a somewhat reduced shape in relation to the geometry of the casting mold. Because of the above-mentioned curvature of the widest faces of the casting mold, the ingot being cast will assume a convex shape in the initial stage of the casting operation. The convexity will gradually reduce until stable conditions with respect to the sump dept in the ingot being cast are stabilized.
The operating manual of a rolling mill specifies that the rolling surfaces should be straight (without any concavity or convexity in the rolling surfaces). To meet this requirement the casting molds have to be designed with a curvature (flexing) of the side walls corresponding to the estimated shrinkage/contraction of the ingot to be cast.
The applicants own EP 0 796 683 B1 relates to equipment for the casting of sheet ingot of the above kind where the side walls are adapted for flexing and are further provided with a stiffening part at their middle region to obtain controlled stiffness and thereby optimal flexure of the mold walls versus the casting speed. This known solution is, however, not designed for casting ingots with different dimensions (size).
When casting ingots or slabs for rolling purposes, which are in the form of large metal blocks with rectangular cross sections, it is normal to employ a special mold for each ingot width and thickness. Mainly because of the close dimensional tolerances required, it is complicated and expensive to produce continuous casting molds. As many different ingot formats are required, it is necessary but uneconomical to keep a corresponding large number of molds in store. Besides, replacing a mold of one dimension with another mold with different dimension is demanding and time consuming.
U.S. Pat. No. 5,931,216 relates to adjustable continuous casting molds for manufacturing continuously cast ingots of different dimensions where the object is to provide an adjustable mold which provides rapid change to the required ingot cross section based on the one and same mold. An important disadvantage with this solution is that the shape of the mold has no means to compensate for casting speed or change of dimension of the mold having in turn bad effect on ingot geometry. Further, this known mold is based on using starter blocks with fixed dimension and design.
In the applicants own International application PCT/NO/09/00309 is shown and described a mold where the disadvantages with the above known solutions are avoided, i.e. where the walls of the mold can be easily adjusted from one dimension to another casting sheet ingots with different dimensions and where at the same time flexing of the walls is possible to compensate for different speeds as well as dimension and alloy composition. However, to change to the desired mold dimension, the starter block is provided with replaceable end pieces on each of its short end sides. This solution is time consuming because the replaceable pieces need to be taken off or added to the short end sides depending on which dimension the mold is adjusted to.